A radio communications system may be largely categorized into an FDD (Frequency Division Duplex) scheme and a TDD (Time Division Duplex) scheme.
According to the FDD scheme, an uplink transmission and a downlink transmission are performed through different frequency bands. According to the TDD scheme, an uplink transmission and a downlink transmission are performed through the same frequency band and at different times. According to the TDD scheme, channel responses are substantially reciprocal to each other. This means that a downlink channel response and an uplink channel response are almost the same in a given frequency region. Accordingly, in a TDD-based radio communications system, a downlink channel response can be acquired from an uplink channel response.
According to the TDD method, an entire frequency band undergoes time division into an uplink transmission and a downlink transmission. Therefore, a downlink transmission by a base station (BS) and an uplink transmission by a mobile station (MS) cannot be simultaneously performed. In a TDD system where an uplink transmission and a downlink transmission are differentiated from each other in the unit of subframes, an uplink transmission and a downlink transmission are performed on different subframes.
A radio communications system includes a base station (BS) which provides a service to neighboring cells. Generally, a terminal or a mobile station (MS) can communicate with the BS when being in a service coverage of the BS. However, when there is an obstacle such as a building or when the terminal or the MS is positioned at a cell boundary, the MS cannot perform a communication with the BS or can perform a communication with an inferior communication quality.
In order to extend the service coverage of the BS, there have been proposed various methods.
One of the various methods is to introduce a relay station (RS) to the radio communications system. The RS is operated as an intermediary between the BS and the MS (or between two MSs and between MS/BS and another RS). More concretely, the RS allows data to be transferred between the BS and the MS far therefrom through two hops or multi hops, not through a single link for direct transfer. This RS may extend the service coverage of the BS, and may enhance a cell boundary performance. Furthermore, the RS may enhance a cell throughput.
The RS was firstly developed from a time division duplex (TDD) radio communications system such as Mobile WiMAX (e.g., IEEE 802.16j/m).
In order to enhance the performance, a Frequency Division Duplex (FDD) radio communications system has started to research about the introduction of a relay station. The FDD radio communications system may include an FDD-based 3GPP (Generation Project Partnership) LTE (Long Term Evolution) system, or a Mobile WiMAX system for supporting FDD, etc.
FIG. 1 illustrates a radio communications system using a relay station.
As shown, the radio communications system includes one or more base stations (BS) 21, 22 and 23 (hereinafter, will be referred to as ‘20’).
Each base station 21, 22 and 23 provides a communications service to a specific geographical area (cell) 21a, 22a and 23a. The cell may be divided into a plurality of areas (sectors). One base station may include one or more cells.
The base stations 21, 22 and 23 indicate fixed stations communicating with terminals 11, 12 and 13 (hereinafter, will be referred to as ‘10’), and may be called eNB (evolved-NodeB), BTS (Base Transceiver System), Access Point, AN (Access Network), etc.
Hereinafter, a downlink (DL) indicates a communication to a terminal from a base station, and an uplink (UL) indicates a communication from a terminal to a base station. In the DL, a transmitter may be part of a base station, and a receiver may be part of a terminal. In the UL, a transmitter may be part of a terminal, and a receiver may be part of a base station.
In uplink transmission, the terminal 11 is operated as a source station, and transmits data to the base station serving as a destination station. In downlink transmission, the base station 21 is operated as a source station, and transmits data to the terminal 11 serving as a destination station.
As shown, the radio communications system may include one or more relay stations 31, 32 and 33 (hereinafter, will be referred to as ‘30’).
As shown, the relay stations 31, 32 and 33 are positioned on an outer periphery area or a shadow area of a cell, and relays data between the base station and the terminal. Here, the base station performs functions such as connectivity, management, control and resource allocations between the relay station and the terminal.
Referring to FIG. 2, the base station performs a communication with the terminal through the relay station.
As shown, the relay station 31 relays the UL and the DL.
In UL transmission, the terminal 11 serving as a source station transmits UL data to a destination station, i.e., the base station 21 and the relay station 31. Then, the relay station 31 relays the UL data of the terminal 11 to the base station 21.
In DL transmission, the base station 21 serving as a source station transmits DL data to a destination source, i.e., the terminal 11 and the relay station 31. Then, the relay station 31 relays data from the source station (the base station 21) to the destination station (the terminal 11).
As shown, the relay station may be implemented in one or plurality in number. That is, the relay stations 32 and 33 may exist between the base station and the terminal 12.
The relay station may adopt a relaying scheme such as an AF (amplify and forward) scheme and a DF (decode and forward) scheme.
Data transferred between the base station 21 and the relay station 31 is called ‘backhaul’ data. The backhaul data may be data by the terminal, or may be data controlled by the base station 21 between the base station and the relay station 31.
In order to prevent the terminal from receiving the backhaul data, a subframe on which the backhaul data is transmitted may be configured not to be heard by the terminal.
In order to prevent the terminal from hearing the backhaul data, used is an MBMS (Multimedia Broadcast/Multicast Service) in 3GPP. The MBMS indicates a streaming service or a background broadcast service or a multicast service provided to a plurality of terminals with using an MBMS bearer service for DL use only. Here, the MBMS may be categorized into a multi-cell service for providing the same service to a plurality of cells, and a single cell service for providing the same service to one cell. In case of the multi-cell service, the terminal may receive, through combinations, the same multi-cell service transmitted from a plurality of cells in an MBSFN (MBMS Single Frequency Network) manner.
However, a legacy terminal which does not support an MBSFN does not perform measurements with respect to a reference signal on a subframe allocated as the MBSFN subframe.
Under this configuration, once a subframe on which backhaul data is transmitted/received between the relay station 30 and the base station 20 is set as an MBSFN subframe, the terminal does not perform measurements with respect to a reference signal on a subframe allocated as the MBSFN subframe.
FIG. 3 illustrates a HARQ process.
As shown in FIG. 3, in the conventional art, a HARQ process is performed for efficient data transfer as follows.
1) First of all, the base station 20 transmits first data to the relay station 30 on a data channel.
2) Then, upon receipt of the first data, the relay station 30 attempts to decode the first data. The relay station 30 transmits a HARQ feedback to the base station 20 according to a result of the decoding. That is, the relay station 30 transmits an ACK signal to the base station in the event of success of the decoding, but transmits a NACK signal to the base station in the event of the decoding. Here, the ACK/NACK signals are transmitted after a predetermined time interval from a reception time point of the data.
3) Upon reception of the ACK signal, the base station 20 determines that the data has been successfully transmitted to the relay station 30. Then, the base station 20 transmits subsequent first data after a predetermined time interval from a reception time point of the ACK signal. On the other hand, upon reception of the NACK signal, the base station 20 determines that the data has been unsuccessfully transmitted to the relay station 30. Then, the base station 20 re-transmits the same first data after a predetermined time interval from a reception time point of the NACK signal, in the same manner or in a new manner.
4) After a predetermined time interval from a transmission time point of the NACK signal, the relay station 30 attempts to receive the first data.
5) Upon reception of the re-transmitted first data, the relay station 30 attempts a decoding process. In the event of success of the decoding, the relay station 30 transmits an ACK signal to the base station after a predetermined time interval from a reception time point of the first data. On the other hand, in the event of failure of the decoding, the relay station 30 transmits a NACK signal to the base station after a predetermined time interval from a reception time point of the first data. The relay station 30 transmits a NACK signal and receives the first data again until it succeeds in decoding the first data. These processes are repeatedly performed.
As can be seen from the above, when transmitting data through an uplink from the relay station 30 to the base station 20, a synchronous HARQ is used.
Here, the synchronous HARQ means that a time interval between data transmissions is constant.
More concretely, when the relay station 30 is to re-transmit data, the re-transmission is performed after a predetermined time from the previous transmission.
For instance, in a 3PP E-UTRA system, a synchronous HARQ is used to transmit data and ACK/NACK, respectively with a time interval of 4 ms. In the 3GPP E-UTRA system, the synchronous HARQ is used not only for the DL, but also for data transmission in the DL from the base station 20 to the relay station 30.
More concretely, in the 3GPP E-UTRA system, a data packet is transmitted on the nth subframe of one frame. And, ACK/NACK with respect to the data packet are transmitted on the (n+4)th subframe. Since the subframe has a time interval of 1 ms, the ACK/NACK signals with respect to the data packet are transmitted with a time interval of 4 ms. If the NACK signal is received on the (n+4)th subframe, a transmitter re-transmits the data on the (n+8)th subframe. The data is re-transmitted with a time interval of 8 ms. Accordingly, this process may be referred to as a HARQ process having a period of 8 ms.
As aforementioned, once a subframe where backhaul data is transmitted/received between the relay station 30 and the base station 20 is established (set) as an MBSFN subframe, a terminal connected to the relay station does not receive a subframe allocated as the MBSFN subframe, and the corresponding subframe does not perform measurements with respect to a reference signal. However, the terminal has to receive a signal transmitted from the relay station on subframes not allocated as the MBSFN.
FIG. 4 illustrates problems occurring when using a synchronous UL HARQ.
As shown in FIG. 4, one frame includes 0˜9 subframes. The upper frames indicate frames corresponding to a downlink (DL) from the base station 20, and a DL from the relay station to the terminal. On the other hand, the lower frames indicate frames corresponding to an uplink (UL) by the terminal, and a UL by the relay station 30. Each subframe has a length of 1 ms. The subframes represented with a dark color in the UL indicate subframes not allocated for MBSFN, and the subframes represented with a bright color in the UL indicate subframes which can be allocated for MBSFN.
As aforementioned, a subframe allocated as an MBSFN for UL backhaul data is not received by the terminal, and a reference signal inside the subframe is not measured by the terminal.
Control information is important information like a synchronous signal and a paging message. Accordingly, a subframe on which the control information is transmitted is not set as an MBSFN subframe. For instance, since 0th, 4th, 5th and 9th subframes in an FDD mode of a 3GPP E-UTRAN system are used for transmission of the above important information, they are not set as MBSFN subframes.
Therefore, the relay station 30 has to relay the important information to the terminals on the subframes, and does not set the subframes as MBSFN subframes.
In this case, the subframes on which the important information is transmitted may collide with subframes for the HARQ process with respect to backhaul data.
For instance, it is assumed that a HARQ process having a period of 8 ms with respect to backhaul data is performed.
As shown in FIG. 4, the relay station 30 receives backhaul data on the 1st subframe (n=1) of frame 0. Then, the relay station 30 transmits a NACK signal with respect to the backhaul data on the 5th subframe (n+4) of frame 0.
Upon reception of the NACK signal, the base station 20 re-transmits the backhaul data on the 9th DL subframe (n+8) of frame 0.
However, the 9th subframe of the DL has to be used for the relay station 30 to transmit the important information to the terminals.
On the 9th subframe of frame 0, the relay station 30 has to transmit the important information and has to simultaneously receive the backhaul data from the base station 20. This may cause collisions. It is impossible to simultaneously perform transmission and reception on the subframe of the DL.
Likewise, on the 7th subframe of frame 2, transmission and reception have to be simultaneously performed. This may also cause collisions.